1. Field of the Invention
The invention relates to a method producing surface-modified materials, such as core-shell materials with the core and the shell(s) being different distinct phases, or materials with a concentration gradient of one or more dopant or substituent element(s) from the surface to the bulk, wherein the method can in particular be applied to produce surface-modified cathode materials for Li batteries with improved performance.
2. Description of the Related Art
The standard cathode materials for Li batteries, such as LiNiO2, LiCoO2, LiNi1-xCoxO2, or LiMn2O4 have some shortcomings during overcharge and at elevated temperatures (e.g. M. M. Thackeray, in: Handbook of Battery Materials, ed. J. O. Besenhard, Wiley-VCh, Weinheim, Germany, 1999, p. 293; G. M. Ehrlich, in: Handbook of Batteries, 3.ed., eds. D. Linden, T. B. Reddy, McGraw-Hill, New York, USA, 2001, p. 35-1). For instance, LiCoO2 and LiNiO2 suffer from decomposition by oxygen loss in the over-charged state in particular at higher temperatures. Especially the decomposition of LiNiO2 is accompanied by a strong self-heating which bears the risk of a thermal run-away of the battery. LiMn2O4 shows a limited performance due to (i) electrolyte decomposition with the formation of H2O and acidic impurities, (ii) disproportionation of LixMn2O4 according to2 Mn3+→Mn4++Mn2+
and dissolution of Mn2+ into the electrolyte, which is promoted by acid-induced delithiation and which increases with increasing temperature, and (iii) Jahn-Teller distortion of discharged Li1Mn2O4.
A possibility to improve the performance of said cathode materials is the doping/partial substitution of Co, Ni, and Mn with other elements, such as Al, Mg, Ti, etc. This doping/substitution may result in a structural stabilization and/or a modification of the electronic properties of the material. The modification may concern the whole bulk of the material, or its surface only. A modification of the surface is, for instance, possible by coating with suitable compounds.
A widely applied coating technique for the production of surface-modified materials in general, and for cathode materials for Li batteries in particular, is the coating of a paste containing the material to be coated, a binder and/or solvents (e.g. T. Aono, M. Kato, N. Inoue, Y. Miyaki, H. Tomiyama, H. Ishizuka, M. Kabutomori, EP 836,238; T. Miyasaka, U.S. Pat. No. 5,869,208). Besides, other coating techniques have been described including, for instance, sputtering, vacuum evaporation, CVD (Chemical Vapor Deposition) (N. Nishida, Y. Shoji, M. Jinno, K. Nishio, T. Saito, JP 08-236114), sol-gel methods, and hydrolysis methods (R. Oesten, U. Heider, A. Kühner, N. Lotz, M. Nieman, DE 19922522; N. Lotz, U. Heider, A. Kuehner, M. Nieman, R. Oesten, WO00/70694).
However, these techniques have some significant drawbacks and limitations. For instance, CVD is not really a suitable process for the coating of large quantities of small particle powders. By sol-gel and related hydrolysis-based processes the choice of materials which can be easily and economically deposited is fairly limited. Also, carbon black can not be coated with sol-gel methods.